This Power Strip is a Fire Starter

1 year ago

A few weeks ago I needed a power strip in my home office. The outlet in question is located behind a filing cabinet so it would need a low profile plug. I jumped on Amazon to buy a surge suppressor strip. That’s when I noticed strips with rotating plugs. I’ve always had some apprehensions about plugs like that, though I could never quite put my finger on why. Looking at the reviews on this particular plug, I found some scary issues. Photos of melted plugs, melted outlets, and cries of “fire hazard”. So I did what any crazy hacker would do – bought two power strips. One with a fixed right angle plug to use in my office, and one with a rotating plug to tear down.

Failed plug – from Amazon reviews

Surge suppressors, power strips, outlet strips, they have many names. Underwriter’s Laboratories (UL) calls them “Relocatable power taps”. They all have several outlets, most have a circuit breaker of some sort inside, and some have circuits for surge suppression. These are some of the most common devices to find in the modern home. Many of our houses were designed and built before surround sound, cable boxes, computers, modems, cell phone chargers, tablet chargers, and all our other modern conveniences. There weren’t as many electrical loads, so the houses didn’t have many outlets. Power strips solve this problem.

After a couple of days, I had my strips in hand. I expected the plug to rotate once – maybe 270 degrees. That would indicate there were wires connecting the rotating head to rest of the plug. Not so – this plug would spin round and round all day long.

Slip rings

Pancake style slip ring

The plug rotates 360 degrees, yet still allows the hot, neutral, and ground wires to stay connected. It does this with the help of slip rings. Slip rings are devices that transmit one or more electrical signals across a rotating joint. Large slip rings can carry 100 or more separate circuits. Most car alternators use slip rings to supply DC excitation current to the rotor. Mechanically they are close cousins to commutators, used to transmit power to DC motors. The difference is that a single slip ring is a single connection, where a commutator is a switching device — in a motor it commutates current to one of several coils. Packaged slip rings used to be expensive, hard-to-find items. Thanks to globalization, you can pick them up at Amazon. A twelve-circuit slip ring will run you $9.99 shipped to your door.

The thing with slip rings is that they are mechanical devices. They wear, they can shatter, and the connections made across them can become intermittent. If you’re powering a circuit through a slip ring, it’s a good idea to put your power supply circuit after the ring, and include enough capacitance to soak up any intermittent connections. I’ve seen older FPGA chips latchup and heat up enough to desolder themselves off a PCB due to power transients caused by a bad slip ring.

Analyzing the Patient

I knew I would be tearing down this connector, but Belkin gave me plenty of information before I got out my screwdriver. Printed on the plug itself was PATENTED USA 5,775,921. The inventor of this particular rotating plug design is Jonie Chou, and Belkin has licensed the patent. Belkin also has their own patents on similar devices, most invented by John Wadsworth. In fact, there are 17 references to Jonie Chou’s patent from Belkin assigned patents. It’s safe to say that Belkin is heavily invested in rotating plugs and outlets.

Hackaday Teardown of t he Belkin Plug

Patent Image

Rear of the plug. Notice the tiny contact patch on the neutral (top) connection.

I kept the ‘921 patent close by as I tore down the rotating plug itself. The 4 screws were easy to remove. The plug didn’t come apart though. It was glued or ultrasonically welded. I had to pry it apart using a utility knife blade as a wedge. The construction of the plug closely follows Figure 1 in the patent. Springs hold the two rings in place: number 5 for neutral and 6 for ground. Hot is a shaft on the plug end which rotates in a sleeve on the cord end. The neutral side has a tiny contact patch on the slip ring – I measured it at 2.24 mm x 1.24 mm. The rings themselves are 0.7 mm thick. Comparing the construction to the failure photographs from the Amazon reviews paints a pretty clear picture. The neutral slip ring forms a resistive connection. This creates heat, softening plastic, and eventually causes arcing as the connections fail. The ground conductor should back up the neutral, but only if the connection completely fails. At that point, the plastic is already softened. I’d be curious to see how many people have problems using these devices with Ground Fault Circuit Interrupter (GFCI) outlets.

Burned due to arcing

Neutral fell completely out of this failed plug

Note the perfectly melted outline of the neutral connection

Another melted plug

Ohms is the Law

How does this happen? Let’s run the numbers with a desk lamp. To keep the things simple, we’ll use a 60 W incandescent light bulb. Ohms law tells us that 60 W is 0.5 A at 120 V. The bulb’s resistance is around 240 Ω. Normally the voltage drop over the wiring is relatively small. But say the resistance at the slip ring is 10 Ω. Now we have a total resistance of 250 Ω. Current drops to 0.48 A, and power to 57.6 W.

4.8 V and 2.3 W of power are now being dropped across that 10-Ω load. That doesn’t sound like much, but where does the heat go? The slip rings heat up. Some of the heat is carried away by the wires, but not all of it. The rest is dumped into the plastic, which starts to soften. The softening plastic allows everything to start moving. The slip rings and contacts slide apart. The plug blades end up at odd angles. Finally, the connection starts to break. A few microns of space opens up between the contacts, which is easy for 120 V to arc over. Arcing starts, dumping more heat into the system, and providing an ignition source. From here one of two things happen – the soft charred plastic moves enough to break the connection, or things catch fire.

The strange thing here is that Belkin knows about the problem. Back in 2009, Belkin recalled 68,700 surge protectors with rotating plugs. The problematic devices had all been manufactured in 2003. Whatever the manufacturing issues were, they were fixed to Belkin’s satisfaction, and the company is still making rotating plugs. My device, in particular, had a manufacturing date of June 2017.

Through all of this, it is important to note that the Belkin device and its plug are Underwriters Laboratories (UL) listed, so the devices have been tested and found to be safe by at least one lab. While Hackaday doesn’t have failure statistics for this particular model, it might be worth UL taking a second look at the design.

This sort of thing can happen with any connection. The difference between the Belkin plug and traditional plugs is that the rotating slip rings provide many more points of failure. AC mains plugs take a lot of abuse. They get yanked out by their cords. They’re banged by furniture. They have to handle corrosion, dust, and gunk building up on their blades. They take heating and cooling cycles as high current loads switch on and off. All this happens over years, even decades in some cases. So why add multiple points of failure to a system which is going to be abused?

Belkin has already been through a recall due to this rotating design. Judging by the photos on Amazon, they might do well to consider revamping the rotating plug completely – before someone loses their shop, their home, or worse. That said, they’re not the only company producing rotating plugs. Searching Amazon for “360 plug” reveals several other licensees of the 5,775,921 patent. Companies such as 360 Electrical are building outlet strips where every outlet rotates. Are any of them safe?

As for me, I installed the fixed plug Belkin surge suppressor in my office. As for the second strip, the rotating plug will soon be cut off and replaced with a plug from the hardware store and the surge suppressor will live out its days powering the computer in my shop.